A color image formed on a piece of paper or other electrographic media by an electrostatic plotter consists of a matrix of dots of selected primary colors. The dots are arranged in vertical image lines or "rasters lines" which combine to form an image. The spacing between dots is dy and the spacing between lines is L.
The color image is a composite image formed by superimposing primary images of the selected primary colors. The primary images must be precisely registered so that the colored dots in each primary image are located in the proper position of the composite image.
Each primary image is formed by affixing dots of colored toner to a piece of paper. The affixation of a dot includes the steps of forming electrostatically-charged dots on the paper, applying toner to the paper, and removing the excess toner. The attraction between the toner and the electrostatic dot causes colored particles in the toner to adhere to the paper. The toner also neutralizes the electrostatic dot, so that the dot will not attract toner of a different color during the formation of other primary images.
Accordingly, the image is formed by positioning electrostatic dots in selected locations. These electrostatic dots are formed by wire styli imbedded in a plotter head in an elongate arrangement. One entire image line of a primary image is formed at a time. The positions of the styli correspond to the positions of the dots in a line of primary image. Those styli in positions where a colored dot is desired are activated by impressing a voltage differential between the desired styli and a backplane positioned near the styli. This voltage causes charge to be stored on a dot of the paper near the vicinity of the styli. The particular styli activated are selected by a write controller.
To form an entire image, the paper and plotter head are placed in relative motion. Typically, either the paper is moved relative to the plotter head or the head is moved relative to the paper. For the moving paper case, a line of the image is formed at time intervals dt.sub.x. The time interval dt.sub.x and paper speed are selected so the distance between lines is L.
The composite color image is produced by forming the corresponding primary images in sequence. These primary images may be formed by a multi-pass system where the same piece of paper is passed through a print station to form a first primary image, rewound, passed through the print station to form a second primary image, rewound, and so on.
Alternatively, a single-pass multi-stationed color printer, as described in a commonly-assigned U.S. patent application, Ser. No. 06/722,497 (Kamas et al.), may be utilized to form the primary images.
A major problem associated with either type of printer is maintaining precise registration of each successively-formed primary image relative to the previously formed image on the paper. Changes in humidity cause the paper to change size and mechanical stress causes the paper to elongate. The dots formed in each primary image must be precisely positioned relative to the dots of the other primary image to form a high resolution composite image.
In systems having fixed heads and moving paper, mechanical paper-guiding systems have been developed to minimize paper wandering and stretching. In systems having movable heads, mechanical servo systems for positioning the head to compensate for paper wandering and stretching have been developed.
These mechanical compensation systems are bulky, expensive, and of limited accuracy.